The present invention relates to an aromatic polyimide/metal composite sheet and an aromatic polyimide film.
An aromatic polyimide sheet has a high rigidity, a high heat-resistance, and a low linear expansion coefficient, and therefore it has been employed for a wide variety of industrial devices such as electronic devices for incorporation into cameras, personal computers, and liquid crystal displays. In these devices, the polyimide sheets are generally used as substrates of flexible circuit-printing boards (for flexible electric circuits, FEC) and substrates for tape automated bonding. The polyimide substrate is used in the form of a composite of an aromatic polyimide sheet (or film) and a metal film (e.g., copper film) combined by an adhesive such as an epoxy adhesive. Most of such known adhesives are somewhat heat resistant, but less heat resistant than the aromatic polyimide. Therefore, these relatively poor heat resistant adhesives give adverse effect to the polyimide/metal composites in their heat resistance.
Therefore, there have been proposed a number of methods for preparing a heat resistant polyimide/metal composite which does not use an adhesive. For instance, a metal is electrically plated on a polyimide film. Also known is a process comprising coating a polyamide acid solution on a metal film (or foil) and heating the coated polyamide acid layer so as to dry and imidize the coated polyamide acid. Further known is a method of pressing a thermoplastic polyimide film onto a metal foil under heating.
U.S. Pat. No. 4,543,295 discloses high temperature polyimide film laminates in which a polyimide film and a metal film are bonded by means of a polyimide adhesive. The polyimide adhesives described therein are Larc-2, -3, and -4 which are a reaction product of BTDA (3,3xe2x80x2,4,4xe2x80x2-benzophenone tetracarboxylic acid dianhydride) and 3,3xe2x80x2-DABP (3,3xe2x80x2-diaminobenzophenone), a reaction product of 2 BTDA/1 PMDA (pyromellitic acid dianhydride) and 3,3xe2x80x2-DABP, and a reaction product of 3 BTDA/1 PMDA and 4,4xe2x80x2-DABP (4,4xe2x80x2-diaminobenzophenone), respectively.
U.S. Pat. No. 5,262,227 describes an aromatic polyimide/metal composite sheet which is produced by pressing a metal foil (or film) onto a polyimide composite sheet composed of plural polyimide films under heating.
It is a main object of the present invention to provide a new polyimide/metal composite which shows a high heat resistant and a high bonding strength between the polyimide film and the metal film.
It is another object of the invention to provide a new polyimide/metal composite which shows a high heat resistance and a high bonding strength between the polyimide film and the metal film and which can be produced under relatively mild conditions.
It is a further object of the invention to provide a new polyimide film which is favorably employable for preparing a heat resistant polyimide/metal composite by combining them with heating under pressure.
It is a still further object of the invention to provide a new polyimide film which has a high heat conductivity and is favorably employable for preparing a heat resistant polyimide/metal composite by combining them with heating under pressure.
There is provided by the invention a polyimide/metal composite sheet comprising a metal film and a polyimide film bonded to the metal film, wherein the polyimide film comprises a polyimide having a recurring unit of the following formula (1): 
There is further provided by the invention a polyimide/metal composite sheet comprising a metal film and a polyimide film bonded to the metal film, wherein the polyimide film comprises 5 to 50 weight % of a polyimide having a recurring unit of the above-mentioned formula (1) and 95 to 50 weight % of a solid filler.